The prior art discloses a rotary vibrating rod which is supported in the center of a membrane. A portion of the rod extends into the container and is dampened by the filling material. The increasing current of the electromagnetic vibration system caused by the dampening can be measured to determine the level of the material. This construction is simple and sturdy. It has, however, the disadvantage that when it vibrates, alternating torques act on the container well via the elastic deformed membranes so that vibrating energy is lost. For this reason the vibration exciting system must be designed accordingly powerful. The apparatus, therefore, does not react on weak dampening by light-weight filling material. As a result it can be applied only for very heavy filling material and there also only under limited conditions.
In order to increase the sensitivity of the instrument, two vibrators were provided instead of one. The phases of the two vibrators are adjusted to each other in a manner that the forces which appear on vibrating compensate each other so that the whole system loses no vibration energy to the container wall.
The prior art also includes a "tuning fork" system wherein two bending vibrators extend parallelly into the container and are excited to bending vibrations in the opposite directions. The alternating torques of the single rods compensate each other. The center of gravity of the system also is located so that the system loses almost no vibrating energy to the container wall and, therefore, has a high sensitivity for light-weight bulk material. The disadvantage of this system is that the material in the container can be jammed between the two vibrating rods which may lead to an erroneous indication. Moreover, the filling materials may build bridges between the two vibrating rods so that they cannot start to vibrate anew when the container is empty.
The prior art also provides vibrating systems where two vibrating elements are used which are concentrically arranged, the outer vibrating element working as a rotary vibrator whereas the inner vibrator element may work as rotary vibrator or a bending vibrator. The two vibrator elements vibrate in opposite directions whereby the alternating torques compensate each other.
Such instruments lose almost no energy to the container wall and have, like the tuning fork system, a high sensitivity also for extreme light-weight filling materials. In contrast to the tuning fork system, only one vibrating element extends into the container so that the problem that bulk goods are jammed between the vibrating rods, and that filling material builds a bridge, does not occur. Nevertheless, these systems, too, in principle consist of two vibrators that function to compensate each other by means of mechanical tension bridges. The precondition of an exact compensation is, therefore, a careful tuning of both single vibrators. This requires comparatively high manufacturing costs. Moreover, there is the disadvantage that the correct function of the instrument can be disturbed by maladjustment caused e.g. by mechanical deformations.
Therefore, it is an object of the present invention to provide a level control instrument comprising a single vibrator system that resists to maladjustment by rough use and which is simple and economical to manufacture, and which reacts at least as sensitively as the known constructions. Moreover, the vibrating rod extending into the container remains accurate even after mechanical deformation.